Control circuit and method for estimating an output current of a transformer

ABSTRACT

A control signal circuit for estimating a current output from a transformer. The control signal circuit includes a converter circuit configured to receive a signal corresponding to a current input to the transformer and output, over a half line cycle, a plurality of values corresponding to the current output from the transformer. The half line cycle corresponds to a period that the transformer is connected to an input voltage. An output current estimator is configured to accumulate the values output from the converter circuit over the half line cycle, determine an average value of the current output from the transformer over the half line cycle, and output the average value of the current output from the transformer over the half line cycle.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present disclosure is a continuation of U.S. patent application Ser.No. 13/246,515 (U.S. Pat. No. 8,704,455), filed on Sep. 27, 2011, whichclaims the benefit of U.S. Provisional Application No. 61/389,655, filedon Oct. 4, 2010. The entire disclosures of the applications referencedabove are incorporated herein by reference.

BACKGROUND

Particular embodiments generally relate to current estimation.

Unless otherwise indicated herein, the approaches described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

The use of electronic devices is popular in stationary and mobileenvironments. One kind of visual display is typically used in bothenvironments. For example, the same kind of visual display is used inelectronic devices from large sign/advertising boards to cellular phonesor portable game players. Energy consumption is a concern in the designof the electronic devices. For example, in the marketplace, anelectronic device that uses less energy may take significant precedenceover other devices.

Light-emitting diodes (LEDs) are being used in displays because of theLEDs' energy efficiency, reliability, low maintenance, and environmentalfriendliness. LEDs have been used in various devices, such as signaldevices (e.g. traffic lights, exit signs, and signboards) as well as insome illumination devices, such as flashlights. Additionally, LEDs maybe used in light sources for general illumination in homes to commercialapplications. LEDs have many advantages, such as long life, energysavings, better quality light output, safety, size, and durability.

A single-stage flyback solution for driving LEDs is used because of theflyback solution's simplicity and cost reduction. The flyback solutionrequires that a current be detected on the secondary side of atransformer of the flyback solution. This increases the amount ofcomponents in the implementation, such as output components, secondaryconstant current control circuits, and an optocoupler are needed on thesecondary side. These components are used to detect the current on thesecondary side and then send the current value back to the primary side.The detected current is used by the primary side to adjust an on and offtime of a switch in the flyback solution.

SUMMARY

In one embodiment, an apparatus includes a transformer comprising aprimary side and a secondary side. A switch is coupled to the primaryside. A control signal circuit is configured to: sample a first currenton the primary side of the transformer; estimate a second current valueon the secondary side of the transformer using the sampling of the firstcurrent on the primary side and a turn ratio of the transformer; andoutput a signal to control a turn on time for the switch.

In one embodiment, the apparatus includes an analog to digital (ADC)converter configured to sample the first current and output a digitalvalue representing a value of a second current; an output currentestimator configured to determine an average of the second current; anaccumulator configured to determine an error current using the averagecurrent and a reference current; and a control signal generatorconfigured to generate the control signal based on the error current.

In one embodiment, a system includes a load coupled to the secondaryside of the transformer and configured to receive a second current basedon the second current values.

In one embodiment, a method includes determining a sampling time tosample a first current on a primary side of a transformer based on aturn on time of a switch coupled to the primary side. The first currentis sampled on the primary side of the transformer. A turn ratio isdetermined between the primary side of the transformer and a secondaryside of the transformer. A second current value on the secondary side ofthe transformer is determined using the sampling of the first current onthe primary side and the turn ratio. A signal is output to control aturn on time for the switch.

In one embodiment, determining the second current value includes:determining a plurality of second current values for multiple samplingsof the first current during a half line cycle of an input signal andaccumulating the plurality of second current values to estimate thesecond current during the half line cycle.

In one embodiment, the second current value is determined based on aturn off time of the switch, the first current sampled during the turnon time of the switch, a frequency of an input signal, and the turnratio.

The following detailed description and accompanying drawings provide amore detailed understanding of the nature and advantages of the presentinvention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts a circuit for estimating current according to oneembodiment.

FIG. 2 depicts a graph showing the relationship between the primarycurrent and the secondary current according to one embodiment.

FIG. 3 depicts the sampling of the primary current and the secondarycurrent according to one embodiment.

FIG. 4 shows a half line cycle T_(half line cycle) according to oneembodiment.

FIG. 5 depicts a more detailed example of the control signal circuitaccording to one embodiment.

FIG. 6 depicts a simplified flowchart of a method for estimating asecondary current according to one embodiment.

DESCRIPTION

Described herein are techniques for a current estimation system. In thefollowing description, for purposes of explanation, numerous examplesand specific details are set forth in order to provide a thoroughunderstanding of embodiments of the present invention. Particularembodiments as defined by the claims may include some or all of thefeatures in these examples alone or in combination with other featuresdescribed below, and may further include modifications and equivalentsof the features and concepts described herein.

FIG. 1 depicts a circuit 100 for estimating current according to oneembodiment. Circuit 100 includes a transformer 102, a switch (SW) 104,an input voltage V_(in), a diode (D), a capacitor (C), a load (R), andan input resistor (R_(i)). In one embodiment, circuit 100 may be asingle-stage flyback solution; however, other circuits may be used wherea primary current (I_(p)) is detected on a primary side and used toestimate a secondary current (I_(s)) on a secondary side. The primarycurrent I_(p) is the current through the primary side of transformer102. The secondary current I_(s) is the current through the secondaryside of transformer 102.

When switch 104 is on, the primary side of transformer 102 is directlyconnected to input voltage V_(in). This results in an increase ofmagnetic flux in a primary winding of transformer 102. The voltageacross a secondary winding of transformer 102 is negative such thatdiode D is reverse-biased (e.g., blocked). At this time period,capacitor C supplies energy to load R. In one embodiment, load R may bea LED/LED string that is being driven by circuit 100, but other loadsmay be used. When switch 104 is off, the energy stored in the primarywinding of transformer 102 is transferred to the secondary winding. Theenergy is then transferred to load R through diode D, which is forwardbiased.

Particular embodiments calculate the primary current I_(p) on theprimary side of transformer 102 and use the calculated primary currentI_(p) to estimate the secondary current I_(s) through the secondary sideof transformer 102. The estimation of the secondary current I_(s) fromthe secondary side is used to adjust a control signal that is used tocontrol switch 104. For example, a control signal circuit 106 is used tooutput the control signal that turns switch 104 on and off. The on andoff time may be varied based on the estimated secondary current. In oneembodiment, the on and off time may be varied to control the powerprovided to the LED/LED string. Using the primary current I_(p) toestimate the secondary current I_(s) reduces the complexity of circuitryon the secondary side.

FIG. 2 depicts a graph 200 showing the relationship between the primarycurrent I_(p) and the secondary current I_(s) according to oneembodiment. The control signal (PWM) shows the turn on time and turn offtime of switch 104. In one example, switch 104 may be implemented usinga MOSFET (not shown) that is turned on and off to close switch 104 andopen switch 104.

An on time T_(ON) is the switch on time. A constant on time T_(ON) isapplied for a half line cycle. A constant T_(ON) time guarantees thepower factor correction of circuit 100. The off time T_(OFF) is definedby the duration of turning switch 104 off to allow the secondary currentI_(s) to reach zero.

A peak current I_(peak, p) of the primary side and the peak current ofthe secondary side, I_(peak,s), may be related based on a turn ratio Nof transformer 102. For example, the following equations may be used todetermine the peak secondary current:

$\begin{matrix}{I_{{peak},p} = {\frac{V_{in}}{L_{p}}T_{ON}}} & (1) \\{I_{{peak},p} = {\frac{V_{out}}{{NL}_{p}}{T_{Off}(t)}}} & (2) \\{I_{{peak},s} = {I_{{peak},p}/N}} & (3)\end{matrix}$

In equation 1, L_(p) is the inductance value of the primary sideinductor, V_(in) is the input voltage, and T_(ON) is the on time forswitch 104. In equation 2, N is the turn ratio between the primary sidewinding and the secondary side winding of transformer 102 and T_(OFF)(t)is the off time of switch 104 in relationship to time. As can be seen inequation 3, the peak secondary current I_(peak,s) is equal to the peakprimary current I_(peak, p) divided by the turn ratio N.

Particular embodiments sample the primary current I_(p) at a time whileswitch 104 is on. For example, the primary current I_(p) is sampled inthe middle of the on period (T_(onå2)) of switch 104. FIG. 3 depicts thesampling of the primary current I_(p) and the secondary current I_(s)according to one embodiment. The primary current I_(p) is sampledhalfway between zero and the peak I_(peak), which is I_(peak, p)/2. Thecurrent may also be sampled at other times. The peak current sampled onthe primary side is equal to:

I _(p)\_(t=T) _(ON) _(/2) =I _(peak,p)/2   (4)

The secondary current I_(s) can be estimated for one switching cycle. Aswitching cycle is a time period of turning switch 104 on and turningswitch 104 off until switch 104 is turned back on. In this case, theprimary current I_(p) goes from zero to the peak current I_(peak,p)during the on time. The secondary current I_(s) goes from the peakcurrent on the secondary side I_(peak, s) to zero during the off time.The total current through load R for the secondary side is equal to thearea in the triangle shown at 302. The area may be equal to:

$\begin{matrix}\begin{matrix}{{I_{{area},s}(t)} = {\frac{I_{{peak},s}}{2}*{T_{OFF}(t)}}} \\{= {\frac{I_{{peak},p}}{2\; N}*{T_{OFF}(t)}(6)}} \\{= {\frac{I_{p}\left( {T_{ON}/2} \right)}{N}*{T_{OFF}(t)}(7)}}\end{matrix} & (5)\end{matrix}$

In equation 5, I_(area, s) (t) is the area shown at 302, I_(peak, s) isthe peak current on the secondary side, and T_(off) (T) is the off time.In equation 6, the current I_(peak,s) is replaced by the currentI_(peak,p)/N. The value for current I_(peak,s) is determined fromequation 3, which is the relationship between the primary side peakcurrent and the secondary side peak current. Equation 4 is used toreplace the primary side peak current I_(peak, p)/2 to derive equation7. In this case, I_(peak, p)/2 is replaced by I_(p)(t) at time=T_(ON/2). Equation 7 is the total current through load R during the offtime. Thus, the area shown at 302 is a function of the primary currentI_(p), the turn ratio, and the on and off times of switch 104.

FIG. 4 shows a half line cycle T_(half line cycle) according to oneembodiment. During the half line cycle, multiple switching cycles areperformed. For example, switch 104 is turned on and off multiple times.For one switching cycle, the primary current I_(p) through the primaryside is shown at 404 and the secondary current I_(s) through thesecondary side is shown at 406. A peak inductor current is shown at 408,which is current I_(peak, p). The secondary current I_(s) is shown at410. For each switching cycle, a value for the secondary currentI_(area,s)(t) may be determined. For example, values for T_(ON/2),T_(OFF), and N may be determined for a period shown at 412. Equation 7is used to determine the secondary current I_(s). The values for thesecondary current I_(s) may be accumulated over the half line cycle todetermine the average secondary current I_(avg,s). For example, thevalues for each switching cycle are estimated and averaged. Thefollowing equations may be used to determine the average secondarycurrent:

$\begin{matrix}\begin{matrix}{I_{{avg},s} = \frac{\sum\limits_{halflinecycle}\; {I_{{area},s}(t)}}{T_{halflinecycle}}} \\{= {\frac{\sum\limits_{halflinecycle}{{I_{p}\left( {T_{ON}/2} \right)}*{T_{OFF}(t)}}}{N*T_{halflinecycle}}(9)}}\end{matrix} & (8) \\{T_{halflinecycle} = {\frac{1}{2\; f}\left( {f\mspace{14mu} {is}\mspace{14mu} {the}\mspace{14mu} {AC}\mspace{14mu} {input}\mspace{14mu} {line}\mspace{14mu} {frequency}} \right)}} & (10)\end{matrix}$

In equation 8, the midpoints of the secondary current I_(s) areaccumulated and divided by the time of the half line cycle to estimatethe average secondary current I_(avg,s). This is for one half linecycle. In equation 9, the area of the secondary current I_(area,s)(t) isreplaced by corresponding equation found in equation 7. The timeT_(halflinecycle) is equal to 1\2 f where f is the frequency of thealternating current (AC) input. Accordingly, the values for thesecondary current I_(s) are accumulated and averaged over the half linecycle time. The secondary current I_(s) can be determined from theprimary current I_(p), the on time of the switch 104, the off time ofswitch 104, the turn ratio, and the frequency of the input signal.

FIG. 5 depicts a more detailed example of control signal circuit 106according to one embodiment. An input circuit 500 provides the inputsignal that is a half wave rectified AC signal. The input signal isinput into the primary side of transformer 102.

Control signal circuit 106 includes an amplifier 502 that receivescurrent Imosfet. Current Imosfet is the primary current I_(p) throughMOSFET 514. Amplifier 502 amplifies current Imosfet and outputs anamplified signal to analog-to-digital converter (ADC) 504. ADC 504samples the signal output by amplifier 502 at different sampling times.For example, sampling time is set at T_(ON)/2, which is the halfwaypoint between the on time of MOSFET 514. The signal is sampled at eachhalfway point in the switching cycles in a half line cycle.

At the sampling time, ADC 504 outputs a digital value into an outputcurrent estimator 506. The output of ADC 504 is the value for thesecondary current I_(area,s). For example, equation 7 is used todetermine the secondary current I_(s). Output current estimator 506accumulates the values for a half line cycle.

The average value I_(avg) is output by output current estimator 506 intoan accumulator 508. This is the average value for the secondary currentI_(s). For example, equation 9 is used to determine the averagesecondary current. Accumulator 508 compares current I_(avg) to areference current I_(ref). The difference is an error signal I_(err).This represents that error in the secondary current I_(s).

The error signal is input into a proportional-integral (PI) controller510. PI controller 510 calculates a duty cycle as a function of theerror signal I_(err). PI controller 510 outputs a PI signal into a pulsewidth modulation (PWM) generator 512. The control signal output by PIcontroller 510 controls the duty cycle of a PWM signal output by PWMgenerator 512. This increases or decreases the on time of MOSFET 514 tominimize the error current I_(err). Increasing or decreasing the on timechanges the value of the secondary current I_(s) by changing the amountof energy transferred from the primary winding to the secondary winding.

FIG. 6 depicts a simplified flowchart 600 of a method for estimating asecondary current I_(s) according to one embodiment. At 602, a samplingtime to sample a primary current I_(p) is determined. For example, thesampling time may be a midpoint of the on time of switch 104.

At 604, a primary current I_(p) is sampled. At 606, an area of thesecondary current I_(s) is calculated. The area estimates the secondarycurrent I_(s) for the off time of switch 104. At 608, the area of thesecondary side of the current is accumulated and averaged for the halfline cycle. This yields the secondary current I_(s). At 610, the controlsignal is adjusted based on the secondary current I_(s) though theinductor on the secondary side of transformer 102.

Particular embodiments provide many advantages. For example, circuitcomplexity on the secondary side is reduced. The complexity oftransferring a current read from the secondary side to the primary sidemay be more complex than the circuitry used by particular embodiments toestimate the secondary current I_(s) using the primary current I_(p).The estimation of the secondary current I_(s) may be accurate enough forproper operation of the circuit.

As used in the description herein and throughout the claims that follow,“a”, “an”, and “the” includes plural references unless the contextclearly dictates otherwise. Also, as used in the description herein andthroughout the claims that follow, the meaning of “in” includes “in” and“on” unless the context clearly dictates otherwise.

The above description illustrates various embodiments of the presentinvention along with examples of how aspects of the present inventionmay be implemented. The above examples and embodiments should not bedeemed to be the only embodiments, and are presented to illustrate theflexibility and advantages of the present invention as defined by thefollowing claims. Based on the above disclosure and the followingclaims, other arrangements, embodiments, implementations and equivalentsmay be employed without departing from the scope of the invention asdefined by the claims.

What is claimed is:
 1. A control signal circuit for estimating a currentoutput from a transformer, the control signal circuit comprising: aconverter circuit configured to (i) receive a signal corresponding to acurrent input to the transformer and (ii) output, over a half linecycle, a plurality of values corresponding to the current output fromthe transformer, wherein the half line cycle corresponds to a periodthat the transformer is connected to an input voltage; and an outputcurrent estimator configured to (i) accumulate the values output fromthe converter circuit over the half line cycle, (ii) determine anaverage value of the current output from the transformer over the halfline cycle, and (iii) output the average value of the current outputfrom the transformer over the half line cycle.
 2. The control signalcircuit of claim 1, wherein the period that the transformer is connectedto the input voltage corresponds to an on time of a switch connected tothe transformer.
 3. The control signal circuit of claim 1, wherein theplurality of values correspond to an area of the current output from thetransformer over the half line cycle.
 4. The control signal circuit ofclaim 3, wherein the converter circuit is configured to calculate thearea of the current output from the transformer over the half line cycleusing (i) a sample of the current input to the transformer, (ii) thehalf line cycle, (iii) a turn ratio of the transformer, and a periodthat the transformer is not connected to the input voltage.
 5. Thecontrol signal circuit of claim 4, wherein, to calculate the area of thecurrent output from the transformer over the half line cycle, theconverter circuit is configured to calculate the area according to${\frac{I_{p}\left( \frac{T_{ON}}{2} \right)}{N}*{T_{OFF}(t)}},$wherein I_(p) corresponds to the sample of the current input to thetransformer, T_(ON) corresponds to the half line cycle, N corresponds tothe turn ratio of the transformer, and T_(OFF)(t) corresponds to theperiod that the transformer is not connected to the input voltage. 6.The control signal circuit of claim 1, further comprising an accumulatorconfigured to (i) compare the average value of the current output fromthe transformer over the half line cycle to a reference current and (ii)output an error signal corresponding to a difference between the averagevalue of the current output from the transformer over the half linecycle to a reference current.
 7. The control signal circuit of claim 6,further comprising a controller configured to calculate a duty cyclebased on the error signal output from the accumulator.
 8. The controlsignal circuit of claim 7, further comprising a pulse width modulationgenerator configured to, based on the duty cycle calculated by thecontroller, selectively control the period that the transformer isconnected to the input voltage.
 9. The control signal circuit of claim1, wherein the converter circuit includes an analog to digitalconverter.
 10. A method for estimating a current output from atransformer, the method comprising: receiving a signal corresponding toa current input to the transformer; outputting, over a half line cycle,a plurality of values corresponding to the current output from thetransformer, wherein the half line cycle corresponds to a period thatthe transformer is connected to an input voltage; accumulating theplurality of values over the half line cycle; determining an averagevalue of the current output from the transformer over the half linecycle; and outputting the average value of the current output from thetransformer over the half line cycle.
 11. The method of claim 10,wherein the period that the transformer is connected to the inputvoltage corresponds to an on time of a switch connected to thetransformer.
 12. The method of claim 10, wherein the plurality of valuescorrespond to an area of the current output from the transformer overthe half line cycle.
 13. The method of claim 12, wherein calculating thearea of the current output from the transformer over the half line cycleincludes calculating the area using (i) a sample of the current input tothe transformer, (ii) the half line cycle, (iii) a turn ratio of thetransformer, and a period that the transformer is not connected to theinput voltage.
 14. The method of claim 13, wherein calculating the areaof the current output from the transformer over the half line cycleincludes calculating the area according to${\frac{I_{p}\left( \frac{T_{ON}}{2} \right)}{N}*{T_{OFF}(t)}},$wherein I_(p) corresponds to the sample of the current input to thetransformer, T_(ON) corresponds to the half line cycle, N corresponds tothe turn ratio of the transformer, and T_(OFF)(t) corresponds to theperiod that the transformer is not connected to the input voltage. 15.The method of claim 10, further comprising: comparing the average valueof the current output from the transformer over the half line cycle to areference current; and outputting an error signal corresponding to adifference between the average value of the current output from thetransformer over the half line cycle to a reference current.
 16. Themethod of claim 15, further comprising calculating a duty cycle based onthe error signal.
 17. The method of claim 16, further comprising, basedon the duty cycle, selectively controlling the period that thetransformer is connected to the input voltage.
 18. The method of claim10, receiving the signal corresponding to the current input to thetransformer and outputting the plurality of values corresponding to thecurrent output from the transformer include using an analog to digitalconverter to receive the signal and output the plurality of values.